Disposable food containers such as plates and platters with outwardly extending portions at their outer edges are known in connection with plastic products. The following patents disclose plastic containers with outwardly projecting portions on their outer flanges: U.S. Pat. No. 3,442,378 to Wolfe, see FIGS. 2 and 3; U.S. Pat. No. 3,268,144 to Gaunt, see FIGS. 3 and 5. Outwardly extending flange features are also seen in pulp molded products. U.S. Pat. No. 1,866,035 to Hart et al. discloses a pulp-molded plate with a bottom portion, a sidewall, a horizontal portion, an upward portion, a horizontal flange, a downward portion and an outwardly directed edge. See page 2, Col. 1, as well as FIGS. 2 and 3 of the '035 patent; note also U.S. Pat. No. 1,748,911 to Chaplin which discloses a pulp-molded plate including a sidewall, a surround, a downturn and an outwardly directed edge of thickened material which acts as a reinforcing annulus. See second Col., page 1, lines 75-86, as well as FIGS. 2 and 3 of the '911 patent. Commercial pulp molded products sometimes utilize geometry including at least a partial horizontal outer annulus around the flange, presumably for ease of trimming the final product which may be trimmed on a horizontal surface after forming. A stiffening outer border as such with a sharp eversion is not an art-recognized method of increasing strength of plastic or pulp molded products and such geometry has not been suggested for pressed paperboard products, discussed below.
Pulp molded articles, after drying, are strong and rigid but generally have rough surface characteristics. They are not usually coated and are susceptible to penetration by water, oil and other liquids. Pressed paperboard containers, on the other hand, can be decorated and coated with a liquid-resistant coating before being pressed by the forming dies into the desired shape. Vast numbers of paper plates and similar products are produced by each of these methods every year at relatively low unit cost. These products come in many different shapes, oval, rectangular or polygonal as well as round, and in multi-compartment configurations.
Many paperboard containers tend to exhibit somewhat less strength and rigidity than do comparable containers made by the pulp molding processes. Much of the strength and resistance to bending of a plate-like container made by either process lies in the sidewall and rim areas surrounding the center or bottom portion of the container. When in use, such containers are often supported by the rim and sidewall while the weight held by the container is located on the bottom portion. Thus, the rim and sidewall generally are placed in tension and flexure when the container is being used.
In plate-like structures made by the pulp molding process, the sidewall and overturned rim of the plate are, cohesive fibrous structures which have sufficient resistance to bending as long as they are not damaged or split. Because the rim and sidewall of the pulp molded containers are of a cohesive, unitary structure, they may be placed under considerable tension and flexure without failing. Plates produced by the pulp molding process do not typically have a continuous functional coating to prevent strength loss during use with hot, moist foods. Internal chemicals can be used to retard moisture and grease absorption. For improved moisture resistance, a secondary film can be laminated to the plate in a separate, post formation, step resulting in a significantly higher cost.
In contrast, when a container is made by pressing a paperboard blank, the flat blank must be distorted and changed in shape and area in order to form the blank into the desired three dimensional shape. This necessary distortion results in seams or pleats in the sidewall and rim, the areas of the container which are drawn in toward the center in press-forming the container, resulting from the decrease in the circumference of the formed container as compared to the blank. Unless considerable care is employed during the process of pressing, these seams or pleats can constitute material lines of weakness in the sidewall and rim areas about which such containers tend to bend more readily than do containers having unpleated sidewalls and rims. Moreover, unless well formed, such seams or pleats will often have a tendency to open or unfold as if attempting to return to their original flat shape. The necessary location of these pleats in the sidewall and rim of pressed paperboard containers places the greatest weakness in the area requiring the greatest strength. Unless carefully formed, such containers typically have been unable to support loads comparable to pulp molded containers of equivalent fiber content. Under tension, flexure or torsion, pleats can exhibit a tendency to open and/or hinge. Accordingly, most known pressed paperboard containers typically have significantly less load carrying ability than do pulp molded containers unless particular care is employed to transform disrupted regions in the plates into substantially integrated fibrous structures during the pressing process. In contrast to pulp molded plates, the pressed containers can easily have a continuous functional coating applied to the paperboard prior to forming, resulting in enhanced performance with hot and moist foods.
More general background with respect to pressed paperboard containers is seen in U.S. Pat. No. 4,606,496 entitled “Rigid Paperboard Container” of R. P. Marx et al.; U.S. Pat. No. 4,609,140 entitled “Rigid Paperboard Container and Method and Apparatus for Producing Same” of G. J. Van Handel et al.; U.S. Pat. No. 4,721,499 entitled “Method of Producing a Rigid Paperboard Container” of R. P. Marx et al.; U.S. Pat. No. 4,721,500 entitled “Method of Forming a Rigid Paper-Board Container” of G. J. Van Handel et al.; and U.S. Pat. No. 5,203,491 entitled “Bake-InPress-Formed Container” of R. P. Marx et al. Equipment and methods for making paperboard containers are also disclosed in U.S. Pat. No. 4,781,566 entitled “Apparatus and Related Method for Aligning Irregular Blanks Relative to a Die Half” of A. F. Rossi et al.; U.S. Pat. No. 4,832,676 entitled “Method and Apparatus for Forming Paperboard Containers” of A. D. Johns et al.; and U.S. Pat. No. 5,249,946 entitled “Plate Forming Die Set” of R. P. Marx et al. The forming section may typically include a plurality of reciprocating upper die halves opposing, in facing relationship, a plurality of lower die halves. The upper die halves are mounted for reciprocating movement in a direction that is oblique or inclined with respect to the vertical plane. The paperboard blanks, after cutting, are gravity fed to the inclined lower die halves in the forming section. The construction of the die halves and the equipment on which they are mounted may be substantially conventional; for example, as utilized on presses manufactured by the Peerless Manufacturing Company. Optionally included are hydraulic controls. See U.S. Pat. No. 4,588,539 to Rossi et al. For paperboard plate stock of conventional thicknesses i.e. in the range of from about 0.010 to about 0.040 inches, it is preferred that the spacing between the upper die surface and the lower die surface is as taught in U.S. Pat. Nos. 4,721,499 and 4,721,500.
As noted earlier, paperboard for disposable pressware typically includes polymer coatings. Illustrative in this regard are U.S. Pat. No. 5,776,619 to Shanton and U.S. Pat. No. 5,603,996 to Overcash et al. The '619 patent discloses plate stock provided with a base coat which includes a styrene-acrylic polymer as well as a clay filler as a base coat as well as a top coat including another styrene acrylic polymer and another clay filler. The use of fillers is common in the art as may be seen in the '996 patent to Overcash et al. In the '996 patent a polyvinyl alcohol polymer is used together with an acrylic emulsion as well as a clay to form a barrier coating for a paperboard oven container. See Column 12, lines 50 and following. Indeed, various coatings for paper form the subject matter of many patents including the following: U.S. Pat. No. 5,981,011 to Overcash et al.; U.S. Pat. No. 5,334,449 to Bergmann et al.; U.S. Pat. No. 5,169,715 to Maubert et al.; U.S. Pat. No. 5,972,167 to Hayasaka et al.; U.S. Pat. No. 5,932,651 to Liles et al.; U.S. Pat. No. 5,869,567 to Fujita et al.; U.S. Pat. No. 5,852,166 to Gruber et al.; U.S. Pat. No. 5,830,548 to Andersen et al.; U.S. Pat. No. 5,795,928 to Janssen et al.; U.S. Pat. No. 5,770,303 to Weinert et al.; U.S. Pat. No. 4,997,682 to Coco; U.S. Pat. No. 4,609,704 to Hausman et al.; U.S. Pat. No. 4,567,099 to Van Gilder et al.; and U.S. Pat. No. 3,963,843 to Hitchmough et al.
Various methods of applying aqueous polymer coatings and smoothing them are known in the art. See U.S. Pat. No. 2,911,320 to Phillips; U.S. Pat. No. 4,078,924 to Keddie et al.; U.S. Pat. No. 4,238,533 to Pujol et al.; U.S. Pat. No. 4,503,096 to Specht; U.S. Pat. No. 4,898,752 to Cavagna et al.; U.S. Pat. No. 5,033,373 to Brendel et al.; U.S. Pat. No. 5,049,420 to Simons; U.S. Pat. No. 5,340,611 to Kustermann et al.; U.S. Pat. No. 5,609,686 to Jerry et al.; and U.S. Pat. No. 4,948,635 to Iwasaki.
Note also the following patents of general interest with respect to forming paperboard containers: U.S. Pat. No. 6,527,687 to Fortney et al. which discloses a cut-in-place forming system with a draw ring and so forth. See Cols. 6-8; U.S. Pat. No. 3,305,434 to Bernier et al. which discloses a paperboard forming apparatus; U.S. Pat. No. 2,832,522 to Schlanger which discloses another paperboard forming apparatus; U.S. Pat. No. 2,595,046 to Amberg discloses still yet another paperboard forming apparatus.
Pressed paperboard containers such as plates, bowls and the like have been improved over the years in terms of strength and processing characteristics. In this respect, container design particularly the placement and configuration of transitions, sidewalls, and brims has been found to impact product performance and influence manufacturing characteristics. One configuration which has enjoyed substantial commercial success is shown in U.S. Pat. No. 5,088,640 to Littlejohn. The '640 patent discloses a disposable plate provided with a smooth outer profile which defines four (4) radii of curvature defined by arcs of the outer portions of the plate. The various radii are selected for enhancing rigidity of the pressed paper plate as compared to other conventional designs made from the same paperboard stock. The flowing arcuate design of the '640 patent offers additional advantages, notably with respect to manufacturing. It is possible to achieve high press speeds with design of the '640 patent, exercise pleating control and maintain product consistency, even when product is formed slightly off-center due to the forgiving tolerances inherent in the design.
Another configuration for pressed paperboard food containers which has also enjoyed substantial commercial success is taught in U.S. Pat. No. 5,326,020 to Cheshire et al. A pressed paper plate configured according to the '020 patent includes three frusto-conical or linear profiled regions about its sidewall and rim. The sidewall region includes a generally annular region flaring upwardly and outwardly from a periphery of a planar inner region and a first frusto-conical, linear profiled region adjoining the annular region with the frusto-conical region sloping outwardly and upwardly from the annular region. The rim region includes an outwardly flaring arcuate annular region adjoining an outer periphery of the first frusto-conical region, and a second frusto-conical region extending generally tangentially from the arcuate annular region. The second frusto-conical or linear profiled region extends outwardly and downwardly at an angle of about 6° to about 12° and preferably about 6° to 10.5° relative to the plane defined by the planar inner region. The rim of the container further includes an outwardly and downwardly flaring frusto-conical lip with a linear profile adjoining an outer periphery of the second frusto-conical region in order to aid in grasping of the paperboard container by the consumer. The downturn and lip provide considerable strength. Additionally, a plurality of radially extending mutually spaced pleats are also formed in the rim region and are internally bonded with portions of the rim region during formation of the paperboard container by a die press. Pressed paperboard containers configured in accordance with the '020 patent are capable of exhibiting very high flexural strength relative to other available containers; however the design is less forgiving in terms of manufacturing tolerances than that of the '640 patent design. In other words, added strength comes at the expense of processability. In still yet other cases, it is seen that an increase in rigidity is achieved by sacrificing Rim Stiffness. One attempt to improve pressware containers was to provide a bowl with a double brim; however, this attempt was not successful due to hinging of the product therebetween resulting in lower strength.
It has been unexpectedly found in accordance with the present invention that the Rigidity and/or Rim Stiffness of a paperboard serving container can be increased for a given configuration by adding pressed rim features including a transition and an outwardly extending evert as hereinafter described.